LIGHTNING CONDUCTORS.
Lightning conductors—first proposed about the middle of the last cen tury by the philosopher Benjamin Franklin as a protection of life and of buildings against the destructive effects of lightning—perform, when prop erly erected, a double office. In the first place, by silently drawing off the accumulated electricity of the clouds, and thus permitting it quietly to pass into the earth, explosive discharges of the electric force are prevented. In the second place, when the accumulated electricity in the atmosphere has acquired a tension sufficient to overcome the resistance of the stratum of air intervening between the earth and the charged cloud, the conductors afford a better path than any adjacent bodies by which the explosive discharge may find its way to the earth. To express this second function of the properly-arranged lightning conductor more accurately, it may be said that it presents a path of least resistance along which the electricity, in finding its way to the earth, will naturally pass in preference to any other.
Afatcria/s.—From the foregoing statements, it is obvious that the light ning rod must be composed of a material of a high electric conductivity, and that it should be of sufficient size to enable it to carry off harmlessly into the earth the largest discharge that may be attracted by it. Of all known materials, the metals are by far the best conductors of electricity, and are, therefore, universally employed in the construction of lightning rods. The metals, however, differ greatly in respect of conductivity; but, for the purpose of this sketch, only copper and iron need receive considera tion. Of these, copper, when quite pure, has about six times the conduct ive power of iron, but, taking the metals as they are found in common use, the former may be said to have about four times the conductivity of the latter. It must be borne in mind, therefore, that an iron rod, to be as effective as one of copper that has been found to be sufficient, must have at least four times the sectional area of the latter.
Dimensions and the proper dimensions of lightning conductors there are some differences of opinion. The French Academy, with reference to iron rods, recommended that such should be made of a square bar from Y., inch to inch in cross-section; if of copper, they could be made much thinner. The shape to be given the rod is a subject of small importance, provided the amount of metal in it is sufficient. Many
rods are manufactured in the form of thin-walled tubes or twisted ribbons of light weight, -under the impression that by giving the metal a greater surface its conductivity is thereby increased correspondingly. This is an erroneous view, as it has been fully demonstrated that electricity, whether of the quality known as lightning or as current electricity, travels through the mass of a conductor, and not over its surface; and the principal reason why the venders of lightning rods recommend such light-weight ribbons or tubes is because they can be sold cheaply. In all cases, a solid bar of square or rectangular section (if of iron) or a twisted cable of wires (if of copper) should be selected in preference to the various ribbon patterns.
Erection of Rods. —With regard to the placing of rods upon a building, it is essential to observe that they should be so arranged as to afford the most efficient protection; this is of more importance than the practice of carrying- them up to a considerable elevation above the roof. Escaping vapor, smoke, or moisture from chimneys and other portions of a building may form ascending currents, which, under suitable conditions, will afford a better path for the lightning than an elevated rod upon another portion of the building; and from this cause there may arise many apparently inex plicable accidents from lightning in the case of buildings having conduct ors. It is generally assumed that a lightning rod properly connected with the earth will protect an area about equal to that of a circle of which the rod is the radius. This will generally be a safe estimate, but it is well to bear in mind that to prevent any injury to the building and its occupants the rod must present a better earth connection than any neighboring body. Should there exist within the building large metallic masses, water- or sewer-pipes, isolated from the rod, but in better earth connection, the light ning may descend the rod to the point nearest the last-named bodies and leave the rod at this point to seek- the earth by the better avenue, destroy ing, in so doing-, whatever may be in its path. On this account it is to be recommended that the rod should always be connected by good metallic connections with all large bodies of metal within the building, such as water- or sewer-pipes.